In the context of packet data transmission networks, the term “multicast” is a network addressing method for the simultaneous delivery of information to a group of destinations. Multicast uses the most efficient strategy to deliver the messages over each link of the network only once, creating copies only when the links to the multiple destinations split (typically network switches and routers). Multicast is often used for streaming media and Internet television applications. Multicast is in use in the existing Internet architecture. IP Source Specific Multicast (SSM) as described in IETF RFCs 3569 and 4607 is one such multicast service. In addition, multicast will be employed in future networks that employ the publish/subscribe (or pub/sub) paradigm. Pub/sub networks will only deliver data to a user when that user has subscribed to that data. Data is published in the network by a publisher and is typically accompanied by some cryptographically generated signature that serves both to identify the publisher and the data itself. In a pub/sub network context, a multicast may arise when multiple subscribers subscribe to the same data stream. The collaboration project known as “PSIRP” is currently working in this area.
A given multicast group is typically identified by both the multicast group address and by the source address. In an IP context, the multicast address is an address selected from a specified range of (IPv6) addresses, whilst the source address is the IP address of the sender. A subscriber subscribes to a multicast group by registering with one of the multicast routers, using the multicast group address and the source address. In the case of a mobile sender whose IP address may change when a handover is performed (between access routers), in order to maintain the multicast during and following handover, the mobile node may be associated with a stable anchor node. In this case, it is an IP address of the anchor node that is the source address of the multicast, with data potentially being tunneled between the sender and the anchor node. Such a solution is presented in C. Jelger and T. Noel. 2002, “Supporting Mobile SSM sources for IPv6 (MSSMSv6)”, Internet Draft—work in progress (expired), (January 2002).
It will be appreciated however that sender mobility may easily render an existing multicast tree inefficient, causing problems similar to triangular routing in Mobile IP. When the sender moves, an anchor node in the multicast tree typically does not, which means that the sender may end up in a location far away from the next hop node in the multicast tree. If the sender continues to send using the existing multicast tree, the situation may result in the unnecessary duplication of traffic (as several next hops may share new links at the new location) and inefficient routing. On the other hand, if the sender requests that the network immediately reorganise the multicast tree so that it will be rooted at the new location of the sender [as described in Thomas C. Schmidt and Matthias Wählisch. 2006a, “Morphing distribution trees—On the evolution of multicast states under mobility and an adaptive routing scheme for mobile SSM sources”, Telecommunication Systems (2006), and Thomas C. Schmidt and Matthias Wählisch. 2006b, “A First Performance Analysis of the Tree Morphing Approach to IPv6 Source Mobility in Source Specific Multicast Routing”, Fifth International Conference on Networking (ICN 2006)], that may result in excessive signalling within the network.
These issues are considered further in T. Sousa, P. Mendes, and E. Monteiro. 2006: “Experimental evaluation of the source specific multicast model in mobile environments. World of Wireless, Mobile and Multimedia Networks”, 2006. WoWMoM 2006. (26-29 Jun. 2006) DOI=10.1109/WOWMOM.2006.44.
In the case of a mobile network comprising a mobile router, the mobile router is responsible for maintaining the stability of IP connections for the mobile nodes belonging to the mobile network. In particular, the mobile router takes care of mobility related signalling on behalf of the mobile nodes. Of course, when a mobile router moves, a number of mobile nodes behind that router move together, multiplying the problems identified above, potentially many times. The problems can be reduced by allowing the mobile nodes to delegate mobility related signalling rights to the mobile router as for example described in WO07134640 and WO03036916.